Catalysts containing mixed oxides of vanadium, phosphorus, and aluminum and/or boron

ABSTRACT

A mixed oxide catalyst precursor comprising vanadium and phosphorus treated in finely divided form with phosphoric acid and water soluble compound of aluminum and/or boron to improve resistance to attrition.

This invention relates to mixed oxide catalysts, and more particularly, to a catalyst comprised of mixed oxides of vanadium and phosphorus, and the preparation and use thereof.

The production of a catalyst comprised of mixed oxides of vanadium and phosphorus is well known in the art, as exemplified by U.S. Pat. Nos. 3,815,892; 4,085,122; 4,304,723; 4,317,778; and 4,351,773. Such catalysts are oxidation catalysts, and are particularly suitable for the preparation of maleic anhydride.

U.S. Pat. No. 4,304,723 discloses a mixed oxide catalyst of vanadium, phosphorus and boron in which the boron to vanadium atomic ratio is from 0.03 to 0.1 to 1. The patentee discloses that the boron compound is introduced into the catalyst at any stage of catalyst preparation prior to calcining, and preferably the boron is introduced during the initial stages of the precursor formation.

In many cases, it is desirable to use such a catalyst in a fluidized bed. As a result, there is a need for a catalyst comprised of mixed oxides of vanadium and phosphorus which not only has the requisite catalyst activity, but which is also resistant to attrition in a fluidized bed.

In accordance with one aspect of the present invention, there is provided a catalyst comprised of mixed oxides of vanadium and phosphorus which has the requisite catalyst activity, and which has an increased resistance to attrition.

More particularly, in accordance with one aspect of the present invention, there is provided a mixed oxide catalyst comprised of vanadium, phosphorus, and a member selected from the group consisting of aluminum, boron and mixtures thereof, which has been prepared by treating a solid catalyst precursor comprising mixed oxides of vanadium and phosphorus with phosphoric acid, and at least one water soluble compound of boron or aluminum, preferably a water soluble compound of boron, followed by drying of the treated solid catalyst. Applicant has found that by treating such a catalyst precursor with phosphoric acid, and a water soluble compound of boron and/or aluminum, followed by drying of the catalyst, there can be obtained a mixed oxide catalyst which has an increased resistance to attrition. Applicant has found that by treating such a catalyst precursor with a phosphoric acid and a compound of aluminum and/or boron followed by drying of the catalyst, the treated particles agglomerate to produce larger catalyst particles and such catalyst particles of increased size have an increased resistance to attrition.

The starting material which is employed in producing a mixed oxide catalyst in accordance with one aspect of the present invention is a catalyst precursor comprised of mixed oxides of vanadium and phosphorus, prepared by procedures generally known in the art, which employ either an aqueous or organic reaction medium. Such catalyst precursor is recovered from the reaction medium by procedures known in the art, such as heating to dryness, filtration and the like.

In accordance with a preferred aspect of the invention, the solid catalyst precursor, comprised of mixed oxides of vanadium and phosphorus, is ground to produce particles having a size of less than 10 microns, and preferably less than 3 microns, preferably by a wet process, such as in a ball mill or high intensity attritor. The grinding or comminuting is accomplished as generally known in the art, with the temperature generally ranging from 20° C. to 100° C., and preferably being in the order of from 50° C. to 95° C.

In this portion of the operation, if desired, additives of a type known in the art to be suitable for use in such a mixed oxide catalyst may be added. Thus, for example, in accordance with a preferred aspect, a hydroxide or other suitable salt of a group IV-B metal, and in particular, zirconium and titanium, may be added to the catalyst at this time.

A catalyst precursor is then recovered from the slurry (if a wet process is used) by vaporizing the water, with spray drying being a preferred technique.

The dry catalyst, which as known in the art predominantly contains vanadium in the tetravalent state, is then calcined in order to convert a portion of the vanadium to the pentavalent state, as well as to remove water of hydration. In general, a partial oxidation of vanadium to the pentavalent state and the removal of water of hydration is accomplished in two separate stages. Thus, for example, the dry catalyst precursor may be heated in the presence of oxygen, preferably as air, at a temperature in the order of from 150° C. to 350° C. in order to convert a portion of the vanadium to the pentavalent state. Such heating is continued for a period of time sufficient to accomplish such results. Such precalcined catalyst is then heated in a non-oxidizing atmosphere to a higher temperature; for example, a temperature in the order of from 400° C. to 550° C., for a duration of time to remove the water of hydration. Although the temperature of 400° C. to 550° C. has been provided for purposes of illustration, it should be apparent that the specific temperature which is employed is dependent upon the method which was originally used for producing the catalyst precursor.

Alternatively, partial oxidation of the catalyst and removal of the water of hydration can be accomplished in a single step by appropriate control of the heating range, nature of the non-oxidizing atmosphere (for example, a mixture of inert gas and oxygen), by procedures known in the art.

The calcined precursor is then comminuted to produce finely divided catalyst, and in particular, a catalyst having a particle size of less than 10 microns, preferably less than 3 microns. As in the previous grinding step, such grinding is preferably accomplished in a wet state, by use of appropriate apparatus, such as a high intensity attritor or ball mill, etc.

During the grinding operation or after the grinding (particle size less than 10 microns), the catalyst is treated with a phosphoric acid and a compound of aluminum and/or boron.

Although the invention is not limited by any theoretical reasons, it is believed that the treatment of the finely divided catalyst (less than 10 microns) with a phosphoric acid results in some solubilization of the catalyst surface, and upon subsequent drying, there is improved binding of particles to each other to increase the resistance to attrition. In addition, it is believed that the phosphoric acid combines with the aluminum and/or boron to form a mixed oxide which further strengthens the agglomerated particles.

The amount of phosphoric acid used in the treatment is coordinated with the amount of boron and/or aluminum compound, as well as the amount of phosphorus in the catalyst precursor so that the ratio of phosphorus to boron used in the treatment is at least 1:1 (preferably a slight excess of phosphorus), and in addition, the ratio of phosphorus in the final catalyst, which is not combined with boron, does not adversely affect the activity of the catalyst (too large a ratio of phosphorus to vanadium can destroy the activity of the catalyst). In addition, there should be sufficient phosphoric acid to accomplish some solubilization of the catalyst precursor during the treatment.

In the final catalyst, excluding the amount of phosphorus combined with the boron and/or aluminum, the ratio of phosphorus to vanadium is from 2:1 to 1:1, with the best results being achieved when the phosphorus to vanadium ratio is in the order of 1:1 to 1.8:1, and most preferably from 1:1 to 1.3:1.

In the final catalyst, the aluminum and/or boron to vanadium ratio is at least 0.10:1, preferably at least 0.15:1, with the ratio generally not exceeding 0.25:1.

The phosphorus forms a mixed oxide with the aluminum and/or boron in an atomic ratio of 1:1. Accordingly, the hereinabove described phosphorus to vanadium ratio is based on phosphorus which is not combined with the aluminum and/or boron as a mixed oxide.

The amount of aluminum and/or boron compound and the amount of phosphoric acid employed in the treatment of the catalyst precursor are coordinated with the amount of vanadium and phosphorus present in the precursor so as to provide a final catalyst having the hereinabove described amounts of phosphorus, vanadium and aluminum and/or boron. In addition, the amount of phosphoric acid is sufficient to provide for some solubilization of the precursor.

The phosphoric acid used in the treatment may be one or more of the phosphoric acids available, including meta-, ortho-, pyro-, poly, P₂ O₅, etc.

The aluminum and/or boron compound may be any one of a wide variety of soluble compounds, with boric acid and hydroxide being preferred.

After treatment with the acid and boron and/or aluminum compound, the treated catalyst particles are dried, resulting in agglomeration of the particles to produce larger particles having an increased resistance to attrition. In general, the larger particles have an average particle size of at least 40 microns, and in most cases, the average particle size does not exceed 200 microns. It is to be understood, however, that the catalyst may be agglomerated into larger particles.

The catalyst is generally formed into a spherical shape in that such shape is preferred for fluidized beds. In most cases, the catalyst is dried into microspherical particles (for example, a size of 40 to 200 microns), with the formation of such microspheres being easily accomplished by the use of a spray drying technique.

After drying of the treated catalyst, the catalyst is generally calcined prior to use thereof.

In accordance with another embodiment, which is less preferred, the first calcination step may be eliminated, and in such case, the uncalcined catalyst precursor is treated with phosphoric acid and the boron and/or aluminum compound, followed by drying and calcination. It has been found that although there is an increase in the attrition resistance, as compared to an untreated catalyst, the omission of the calcination step, prior to the treatment with phosphoric acid and the boron and/or aluminum compound, produces a catalyst which is less resistant to attrition than a catalyst which is calcined prior to the treatment.

Although the hereinabove described process wherein the catalyst precursor is calcined to both partially oxidize the vanadium and remove water of hydration, followed by treatment of finely divided catalyst and drying increases the resistance to attrition, there is some loss of catalyst activity, as compared to treatment of the uncalcined catalyst.

Accordingly, in accordance with a particularly preferred embodiment, a mixture of calcined and uncalcined precursor, in finely divided form, is treated as hereinabove described. Since treatment of uncalcined catalyst with acid and boron and/or aluminum compound retains activity, with some increase in attrition resistance, and treatment of calcined catalyst greatly increases attrition resistance, with some loss of catalyst activity, in accordance with the preferred embodiment, a mixture of calcined and uncalcined catalyst, in finely divided form, is treated with acid and boron and/or aluminum compound, followed by drying to produce a final catalyst having a desired balance of attrition resistance and catalyst activity. Thus, an increase in the quantity of uncalcined catalyst in the mixture which is treated increases activity and reduces resistance to attrition and vice versa. By varying the ratio, there can be achieved a desired balance between catalyst activity and resistance to attrition. In general, if a mixture is used, the ratio of calcined precursor to uncalcined precursor is from 10:1 to 1:10, and preferably from 4:1 to 1:4.

As hereinabove indicated, the catalyst precursor comprised of mixed oxides of vanadium and phosphorus may be prepared by procedures generally known in the art, including reaction in either aqueous or organic medium. Thus, as known in the art, the vanadium component of the catalyst precursor may be obtained by use of either a tetravalent vanadium salt or by the use of a pentavalent vanadium compound which can be reduced in situ to a tetravalent vanadium salt.

As representative examples of suitable compounds, there may be mentioned vanadium tetrachloride, vanadium dioxide, vanadium oxydibromide, etc., all of which are tetravalent salts; and vanadium pentoxide (which is preferred), vanadium oxytribromide, vanadium oxytrichloride, etc., all of which are pentavalent vanadium compounds.

As the source of phosphorus in the catalyst precursor, there may be employed phosphorus acid, phosphoric acid, such as metaphosphoric acid, triphosphoric acid, pyrophosphoric acid, and the like. As known in the art, vanadium and phosphorus compounds are reacted in either an aqueous or organic system, under non-oxidizing conditions so as to maintain the vanadium in the tetravalent form, or in the alternative, under reducing conditions, when a pentavalent vanadium compound is employed so as to convert the vanadium to tetravalent form, in situ.

In general, as known in the art, the phosphorus and vanadium compounds are reacted in an acid solution, preferably one which has reducing properties, such as hydrochloric acid.

The procedures for producing the catalyst precursor comprised of mixed oxides of vanadium and potassium are well known in the art, for example, as described in U.S. Pat. No. 4,085,122, and the other patents and, therefore, no further details in this respect are deemed necessary for a complete understanding of the invention.

Although the catalysts produced in accordance with the invention may be employed as a catalyst in a wide variety of oxidation reactions, the catalyst is particularly suitable for producing maleic anhydride, and in particular, in a fluidized bed.

As generally known in the art, n-butane may be oxidized to maleic anhydride in the presence of fluidized catalyst by reaction of n-butane with oxygen at a temperature in the order of from 320° C. to 500° C., and preferably from 360° C. to 460° C. The reaction is accomplished with an excess of oxygen, with the oxygen preferably being provided in combination with an inert gas, such as in air, with the oxygen to butane ratio ranging from 15:1 to 1:1 and preferably from 10:1 to 2:1 by weight. It is to be understood, however, that although butane is a preferred feed, as known in the art, saturated or unsaturated C₄ to C₁₀ hydrocarbon or mixtures thereof are generally suitable as feeds for producing maleic anhydride; e.g., n-butanes, 1,3-butadiene, or a C₄ cut from a refinery, with n-butane being particularly preferred.

In the following examples, the resistance to attrition of a catalyst was tested by a procedure similar to the one described in U.S. Pat. No. 4,010,116 (column 3). In the test, the fines (particles with sizes below 20 microns), generated by one jet of air with close to sonic velocity, and which impinges vertically upwards into a known amount of catalyst, are retained and weighed between the 30th and 90th minute from the beginning of the test. The fines are recovered, as described in U.S. Pat. No. 4,010,116, and the figures representing the attrition rate (AR) are calculated as weight percent fines generated in the period of one hour (30th to 90th minute) from the particular catalyst tested and at the conditions specified.

Although there is no quantitative correlation between the attrition rate as calculated herein, and the manner in which a catalyst will actually perform in a plant, in order to provide a frame of reference as to a desired resistance to attrition, catalysts (other than unsupported mixed oxides of vanadium and phosphorus) which are commercially used and known to be resistant to attrition in a fluidized bed were tested by the same procedure in order to determine the attrition resistance of such catalysts. In testing three different commercially available catalysts of such type, it was found that the AR ranges from 2 to 26, with a lower value for the AR signifying a more attrition resistant catalyst.

EXAMPLE 1

One thousand grams of dried complex of mixed oxide of vanadium and phosphorus (VPO) prepared according to U.S. Pat. No. 4,085,122 (Example 1) was mixed with 1000 g H₂ O and 235 g of a paste of hydrated zirconium hydroxide (approx. 85 wt% water content) and introduced into a high intensity ball mill. The "Attritor 1-S" laboratory model manufactured by Union Process, Inc., Akron, Ohio was used during this work.

The grinding media consisted of 40 lbs. of stainless steel balls with 3/16" dia.

1. Grinding-1--The operation was carried out for one hour at rotation velocity of the shaft of approximately 370 rpm. The dissipation of mechanical energy caused the temperature of the medium to increase within one hour to approximately 80° C. although no heating medium was circulated through the jacket of the attritor. A sample of the slurry showed that no particles existed with diameters larger than 0.5 μm.

2. Recovery--The slurry was removed from the attritor and spray dried. Most of the material recovered consisted of microspheres with particle sizes of 40-200 μm.

3. Calcination--The material recovered from spray-drying was heated gradually to 470° C. and maintained at this temperature for 6 hours. An atmosphere of N₂ was maintained in the oven, during the calcination.

4. Grinding-2--One thousand grams of material recovered from the previous step were mixed with 1000 g H₂ O and introduced into the attritor. The grinding was started to reduce particle size. Then a solution of 183 g H₃ PO₄ (85%) and 69 g boric acid in 300 g H₂ O was added. No cooling water was circulated through the jacket. After three hours of operation, a sample of the slurry showed that all particles had sizes under 0.5 μm.

5. The slurry was drained from the attritor and spray dried. The microspherical material with diameters of 40-200 μm was recovered and submitted to (6) calcination at the conditions described for step (3).

In order to evaluate the effects of the above treatment, samples of microspherical material were recovered after both steps (3) and (6) and were submitted to the attrition test described earlier. The results are presented in Table 1 (1 and 1A, respectively).

EXAMPLE 2

The activity of the catalysts was tested in a fluid bed reactor. The reactor was made of a Pyrex tube (4.6 cmID) provided at the lower part with a frit of sintered glass and placed inside a vertical cylinder heated electrically. Air and n-butane are metered via mass flow controllers and fed below the frit. The reactor effluent is water washed in two bubblers in series and its flow rate is measured. The composition of the feed and vent gases were determined by gas chromatography.

The performance of the catalysts was determined on the basis of the weight of butane fed to the reactor, amount of maleic anhydride (MA) recovered in the wash-water (acidimetry) and the amount of butane in the off gases (volume and concentration) during a specified period of time as: ##EQU1##

In order to provide a basis for comparison, the following conditions were maintained during the activity tests:

Reaction Temperature: 390°-420° C.

n-Butane conc. in feed: 3.5-4.5 vol%

Air flow rate: 1 L/min measured at STP

Catalyst loaded to reactor: 0.250 Kg

A sample of catalyst obtained after step (6) of Example 1 was introduced in the reactor and tested as described here. The reaction conditions and the results are recorded in Table 1.

EXAMPLE 1.A

For comparison purposes, microspherical catalyst obtained after step (3) calcination was used in the activity test according to Example 2. The results are recorded in Table 1.

EXAMPLE 3

This example is a less preferred embodiment in that the catalyst is not calcined prior to treatment.

Preparation was performed in the conditions of Example 1 with the difference that steps (1-3) were omitted. In step (5), 1000 g of dried VPO complex and 235 g of a paste of hydrated zirconium hydroxide were mixed with 1000 g H₂ O and ground as described in Example 1. The microspherical catalyst recovered after step (6) was used for the attrition tests. The activity test was performed as in Example 2. The results are recorded in Table 1.

EXAMPLE 4

In another preferred way of carrying out the manufacture of the catalyst, the procedure outlined in Example 1, steps (1-3) was followed as described.

In step (4), the materials fed to the attritor consisted of 500 g catalyst recovered from step (3) and 500 g dried VPO complex obtained according to U.S. Pat. No. 4,085,122 (Example 1), mixed together with 1000 g H₂ O. The procedure outlined in steps (4-6) was then followed. The resistance to attrition and the chemical performance of the microspherical catalyst which was obtained was tested as outlined before. The results are given in Table 1.

EXAMPLE 5

The performance of the catalyst prepared according to Example 4 was tested further. In a metal reactor with an internal diameter of 5.1 cm, 1000 g of microspherical catalyst were introduced. In the conditions of the reaction, the height of the fluid bed was approximately 60 cm. The reactor was provided with internal gas redistributing devices. The results of the test are recorded in Table 1.

EXAMPLE 6

500 g of dried complex of VPO prepared according to U.S. Pat. No. 4,085,122 (Example 1) and 500 g of the same complex calcined at 470° C. in nitrogen were mixed with 1000 g H₂ O and introduced into the attritor; the grinding was started. Then a solution of 183 g H₃ PO₄ (85%) and 69 g boric acid in 300 g H₂ O was added. No cooling water was circulated through the jacket. After three hours of operation, a sample of the slurry showed that all particles had sizes under 0.5 μm.

The slurry was drained from the attritor and spray dried.

The microspherical material with diameters of 40-200 μm was recovered and submitted to calcination at the conditions described for step (6) of Example 1.

In order to evaluate the effects of the above treatment, samples of the calcined material were submitted to the attrition test described earlier. The results are presented in Table 1.

EXAMPLE 7-9

The catalyst of Example 6 was tested in a fluid bed by feeding several hydrocarbons.

The tests are listed in Table 2.

                                      TABLE 1                                      __________________________________________________________________________                 SA     nC.sub.4 in feed                                                                     Conversion                                                                           Select                                                                              Yield                                      Example Number                                                                          AR m.sup.2 /g                                                                        Temp.                                                                              Vol. %                                                                               %     mole %                                                                              mole %                                                                              Observations                          __________________________________________________________________________     1        2.1                                                                               0.4                                                                               404 3.5   36.9  68.3 25.2 B/V = 0.21                                                                     P/V = 1.38                            1-A      41 30 409 3.5   56.3  56.3 43   After step 3                          3        7.5                                                                               24.3                                                                              390 3.5   75    57   42.8 B/V = 0.21                                                                     P/V = 1.38                            4        3.5                                                                               16.5                                                                              397 3.5   85.1  51.7 44.0 B/V = 0.21                                                                     P/V = 1.38                            5        3.5                                                                               16 395 4.5   86.1  60.5 52.1 Air flow rate:                                                                 4.0 1/min.                            6        4.0                                                                               14.3                                                                              412 4.1   78.5  54.5 42.8 --                                    __________________________________________________________________________      NOTE:                                                                          B/V = atomic ratio Boron/Vanadium                                              P/V = atomic ratio Phosphorus/Vanadium                                   

                  TABLE 2                                                          ______________________________________                                                                Hydro-                                                                         carbon Hydro-                                           Ex-                    concen-                                                                               carbon Select-                                                                               MA                                 am-                    tration                                                                               conver-                                                                               ivity  yield                              ple  Hydro-    Temp.   in feed                                                                               sion   to MA  MOL                                N°                                                                           carbon    (°C.)                                                                           (vol. %)                                                                              (mol %)                                                                               (mol %)                                                                               %                                  ______________________________________                                         7    1 butene  390     4.7    99.0   56.6   56.0                               8    cis-2-butene                                                                             393     4.4    99.1   59.0   58.5                               9    1,4-      385     4.5    100.0  57.0   57.0                                    butadiene                                                                 ______________________________________                                    

The present invention is particularly advantageous in that it is possible to provide a catalyst comprising mixed oxides of vanadium and phosphorus which is highly resistant to attrition, and which has the requisite catalyst activity for accomplishing oxidation reactions, and in particular, oxidation of hydrocarbon to maleic anhydride. Moreover, by proceeding in accordance with the invention, it is possible to obtain a variation of the catalyst activity, and the resistance to attrition, by adjusting the amounts of calcined and uncalcined precursor present in the mixture which is subjected to the treatment.

The use of both a phosphoric acid and an aluminum nad/or boron compound in treatment of the precursor provides a further increase in the resistance to attrition.

These and other advantages should be apparent to those skilled in the art from the teachings herein.

Numerous modifications and variations of the present invention are possible in light of the above teachings and, therefore, within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 

What is claimed is:
 1. A composition of matter, comprising:a mixed oxide catalyst comprising vanadium, phosphorus and a member selected from the group consisting of aluminum, boron and mixtures thereof, said catalyst having a phosphorus to vanadium atomic ratio, based on phosphorus uncombined with said member, of from 1:1 to 2:1, an atomic ratio of said member to vanadium of at least 0.10:1, said catalyst having been prepared by treating dried solid catalyst precursor comprising mixed oxides of vanadium and phosphorus in finely divided form with a solution of phosphoric acid and a member selected from the group consisting of water soluble compounds of aluminum, boron and mixtures thereof; and drying of the treated solid catalyst precursor to provide a solid catalyst.
 2. The catalyst of claim 1 wherein the finely divided catalyst precursor has an average particle size of less than 10 microns.
 3. The catalyst of claim 2 wherein said member is boron.
 4. The catalyst of claim 3 wherein at least a portion of the catalyst precursor prior to treatment has been calcined to remove water of hydration and provide a partial oxidation of vanadium to the pentavalent state.
 5. The catalyst of claim 3 wherein uncalcined dried catalyst precursor, in finely divided form, is subjected to the treatment.
 6. The catalyst of claim 3 wherein the dried catalyst precursor, in finely divided form, which is subjected to treatment is a mixture of calcined and uncalcined catalyst precursor, the ratio of calcined to uncalcined catalyst precursor in such mixture being from 10:1 to 1:10.
 7. The catalyst of claim 6 wherein the ratio of boron to vanadium is no greater than 0.25:1.
 8. The catalyst of claim 6 wherein the dried catalyst has an average particle size of at least 40 microns.
 9. The catalyst of claim 8 wherein the dried catalyst is microspherical, and has an average particle size of from 40 to 200 microns.
 10. The catalyst of claim 2 wherein the catalyst precursor, in finely divided form, which is subjected to treatment is a mixture of calcined and uncalcined catalyst precursor, wherein the ratio of calcined precursor to uncalcined precursor is from 10:1 to 1:10.
 11. The catalyst of claim 10 wherein the dried catalyst is in microspherical form, and has an average particle size of from 40 to 200 microns.
 12. The catalyst of claim 1 wherein the catalyst is in a form, and has a resistance to attrition suitable for use in a fluidized bed.
 13. The catalyst of claim 9 wherein the catalyst has a phosphorus to vanadium ratio, based on phosphorus uncombined with said member, of from 1:1 to 1.3:1.
 14. In a process for preparing a mixed oxide catalyst, the improvement comprising:treating a dried catalyst precursor comprising mixed oxides of vanadium and phosphorus, in finely divided form, with a solution of phosphoric acid and a compound selected from the group consisting of water soluble compounds of boron, aluminum and mixtures thereof in an amount sufficient to provide a mixed oxide catalyst having a phosphorus to vanadium ratio, based on phosphorus uncombined with a member selected from the group consisting of aluminum, boron and mixtures thereof, of from 1:1 to 2:1 and a ratio of said member to vanadium of at least 0.10:1; and drying of the treated catalyst precursor to produce said mixed oxide catalyst.
 15. The process of claim 14 wherein the finely divided dried catalyst precursor has an average particle size of less than 10 microns.
 16. The process of claim 15 wherein the compound is a water soluble compound of boron.
 17. The process of claim 16 wherein the finely divided dried catalyst precursor which is subjected to treatment is a mixture of calcined and uncalcined catalyst wherein the ratio of calcined to uncalcined catalyst is from 1:10 to 10:1.
 18. The process of claim 17 wherein the boron to vanadium ratio is no greater than 0.25:1.
 19. The process of claim 17 wherein the treated catalyst precursor is dried to produce a catalyst having a particle size of at least 40 microns.
 20. The process of claim 19 wherein the treated catalyst precursor is spray dried to produce microspherical catalyst particles having an average particle size of from 40 to 200 microns.
 21. A process for producing a mixed oxide catalyst for use in a fluidized bed, comprising:calcining uncalcined dried catalyst precursor comprising mixed oxides of vanadium and phosphorus to remove water of hydration and provide a partial oxidation of vanadium to the pentavalent state; providing a mixture of uncalcined dried catalyst precursor and the calcined catalyst precursor, in finely divided form, said mixture having an average particle size of less than 10 microns; treating said mixture with a solution of phosphoric acid and a compound selected from the group consisting of water soluble compounds of boron, aluminum and mixtures thereof in an amount sufficient to provide a mixed oxide catalyst having a phosphorus to vanadium ratio, based on phosphorus uncombined with a member selected from the group consisting of aluminum, boron and mixtures thereof, of from 1:1 to 2:1 and a ratio of said member to vanadium of at least 0.10:1; drying the treated mixture to produce a dried catalyst having an average particle size of at least 40 microns; and calcining said dried catalyst.
 22. The process of claim 21 wherein the mixture of calcined and uncalcined catalyst precursor which is treated has a calcined to uncalcined catalyst precursor ratio of from 1:10 to 10:1.
 23. The process of claim 22 wherein the compound is a water soluble compound of boron and the boron to vanadium ratio is no greater than 0.25:1. 